Add blinking colon in clock mode

[domneedham]

Looking at the firmware in the C version, the colon has been implemented to easily determine what quarter of the minute it is. I think this is great and so will add an option to support this too.

• Add blinking colon in clock (toggle with double click bottom button?)
• Blink top 0-15 seconds
• Blink bottom 15-30 seconds
• Blink alternate 30-45 seconds
• Blink both 45-60 seconds